Magnetic coating compositions containing magnetic particles, such as .gamma.-Fe.sub.2 O.sub.3 pigment, are widely used for producing magnetic record carriers, in particular, magnetic disks and magnetic tapes. After the application of the magnetic coating composition onto the carrier, the majority of the solvent evaporates. During or after the application of the coating composition and the evaporation of the solvent, there takes place an orientation of the magnetic particles, the curing and polishing of the record carrier, if necessary a lubrication of the surface, and finally a test for the magnetic characteristics of the record carrier. The quality of the magnetic coating composition is of decisive importance for the quality of the magnetic record carrier.
The magnetic coating composition is generally made by using methods of paint production. Apart from precisely observing the chemical composition, it is of great importance to implement a complicated mechanical processing, for example ball milling the coating material to disperse the magnetic particles within the resin solvent fluid as good as possible. This, and the physical-chemical characteristics of the components of the coating composition, as well as their interaction, determine the internal structure of the coating composition, with the degree of dispersion and of agglomeration of the individual particles playing an important part.
For these characteristics which strongly influence the quality of the finished record carrier there do not yet exist any direct measuring means. The viscosity of the coating composition which very much depends on the internal structure influences the processing characteristics. Electrostatic and magnetostatic forces between the magnetic particles can cause aging which only becomes visible later in the finished record carrier.
The quality of the coating composition is at present mostly tested in production in that a control record carrier is made which substantially corresponds to a finished record carrier, and that the latter is then tested for quality. Further use of the coating composition depends on this check. It is obvious that such a test is very time-consuming, expensive and imprecise since immediately before, and during the application of the coating composition onto the carrier, no measure of the quality of the coating composition is available.
Decisive factors for the quality of the magnetic coating composition are the degree of dispersion or of agglomeration, respectively, the particle density, and the viscosity. Furthermore, the stability of the composition place a part which strongly depends on time, i.e., what is the stability of the coating composition during a predetermined period or in other words, does the coating composition age prematurely, and if so, to what extent?
In Applicant's co-pending patent application Ser. No. 534,471 Filing Date Sept. 21, 1983, now U.S. Pat. No. 4,651,092, a method and an arrangement is described for characterizing magnetic coating compositions consisting of a dispersion of magnetic particles, particularly .gamma.-Fe.sub.2 O.sub.3 pigment, in a liquid resin-solvent mixture. In accordance with this known solution the coating composition is exposed to an alternating magnetic field of variable frequency, the signal induced by the alternating magnetic field is received from the coating composition, and consequently the susceptibility thereof is measured as a factor of the variable frequency, and evaluated with respect to the degree of dispersion, the particle density, and to the viscosity of the coating composition. This known method and apparatus allows the measuring and controlling of the quality of the coating composition.
Nevertheless this known method and arrangement is somewhat limited in its measuring flexibility. It is not possible to change the composition during measurement for measurement purposes and therefore also it is not possible to change the degree of dispersion even for a very limited period of time.